It is known to subject a heavy hydrocarbon feedstock to a hydrocracking process which makes use of a series of catalysts.
From U.S. Pat. No. 4,435,275, it is known to hydrocrack a hydrocarbon feedstock using typically mild hydrocracking conditions by passing the feedstock firstly over a bed of an amorphous hydrotreating catalyst and subsequently, without intermediate separation or liquid recycle, passing the hydrotreated feedstock over a zeolitic hydrocracking catalyst. The zeolite in the hydrocracking catalyst can be selected from faujasite, zeolite X, zeolite Y, mordenite or zeolite ZSM-20.
The products of lower average molecular weight and lower boiling point thus obtained by hydrocracking include gaseous material, i.e. in general C.sub.1-4 hydrocarbons, naphtha and a middle distillate fraction, i.e. a kerosene fraction and a gas oil fraction. It is evident that the cut between hydrocracked products may be made at various boiling points.
Since the gaseous products are not very much wanted and since there is an increasing demand for middle distillates, it would be advantageous to have a two-stage process available for converting hydrocarbon oils that shows a considerable selectivity towards middle distillates and a low gas make.
It has now been found that a good yield of middle distillates and low gas make can be obtained if a hydrocarbon oil containing a relatively low amount of nitrogen is passed over a catalyst system comprising a series of a catalyst which comprises a wide pore zeolite and an amorphous silica-alumina containing catalyst.